The present invention relates to a single longitudinal mode semiconductor laser, and more particularly to a semiconductor laser having a distributed feedback structure.
Optical fiber communication has now reached a stage of practical application as enabled by the reduction of losses in optical fibers and the improvement of semiconductor laser performance. The attraction of optical fiber communication lies above all in the long-distance high-capacity transmission it permits. Optical fibers, however, vary in transmission velocity with optical wavelength (wavelength dispersion), and semiconductor lasers used as light sources oscillate during high-speed modulation in multiaxial modes (in a plurality of modes having different wavelengths) and are thus susceptible to signal waveform degradation in high-capacity long-distance transmission. To solve these problems, many attempts are under way to develop a single longitudinal mode semiconductor laser oscillating in a single longitudinal mode even during high-speed modulation.
Single longitudinal mode semiconductor lasers include distributed-feedback semiconductor lasers (DFB lasers). A DFB laser usually has a structure in which a grating is formed in an optical guide layer provided in the vicinity of an active layer. The DFB laser, utilizing periodic variations of the refractive index resulting from the presence of the grating, oscillates in a single longitudinal mode around the Bragg wavelength of this grating. In order to achieve satisfactory single longitudinal mode oscillation, a DFB laser requires efficient coupling between the grating and the optical field. Therefore, the most suitable position where the grating is to be formed is an area in which there is a strong optical field. In other words, as light rays propagate through the active layer and the optical guide layer, their intensity is the greatest in or around the area between the active and optical guide layers, and it is desirable to form the grating in this area. However, in most of the previously produced DFB lasers, the grating is formed on the side of the optical guide layer farther from the active layer, where the optical field is relatively weak. (See, for example, T. Matsuoka et al., "CW Operation of DFB-BH GaInAsP/InP Lasers in 1.5 .mu.m Wavelength Region", ELECTRONICS LETTERS, Vol. 18 (1982), No. 1, pp. 27-28). Accordingly, the index K which represents the coupling efficiency between the grating and the optical field is no greater than approximately 40 cm.sup.-1, resulting in the problem that increasing the injected current would invite oscillation at the Fabry-Perot mode instead of oscillation in a single wavelength.